Basic Chem: Predicting Reactions

Factual Questions
1

This question has always bugged me: Given C2H2 + O2, predict the products and balance the equation. Now, aren’t there numerous solutions? I could yield:
a) C6H10O6 (sugar)
b) CH4 + H2O
c) CO+CO2+H2O

…and so on! Since it is initially unbalanced, aren’t all of these possible solutions? If not, how do you know? I am tutoring a 11th grader in chem, and most of this is old-hat, but this NEVER sat right with me. Keeping it simple, I assumed:
C2H2 + O2 → C + H2 + O2

In the lab, doesn’t this highly depend on T,P of the experimental conditions which decide the outcome? Also, since there is no indication if I am putting energy into the experiment or not, I just might be doing electrolysis to yield only elements, right? - Jinx :confused:

2

What you’re talking about it oxidation of acetylene. The oxygen molecules will donate electrons to the carbon and hydrogen, and in the process bonding with them. The correct equation is:

2 C2H2 + 4 O2 -> 4 CO2 + 2 H2O

This is the most likely solution. However, in the case of incomplete oxidation, you could have:

C2H2 + 2 O2 -> 2CO + H2O + O-

And indeed, carbon monoxide buildup is a concern when burning hydrocarbons in an enclosed space.

Creating glucose, though, requires a lot of energy, more energy than creating CO2 and H2O. I tried coming up with the thermodynamic numbers to support that, but thermo was never my best subject in chemistry. I think Chronos could do it more justice than I. Also, glucose has six carbons, requiring multiple energy requiring steps, whereas oxidation of acetylene occurs in one neat step.

Vlad/Igor

3

Well, I would do this:

C[sub]2[/sub]H[sub]2[/sub] + 5O[sub]2[/sub] -> 4CO[sub]2[/sub] + 2H[sub]2[/sub]O, because that’s how they’ll react. I don’t know how well electrolysis of ethyne would work, because hydrocarbons and triple bonds are pretty reactive, and the electricity necessary for electrolysis might cause it to burn.

4

What are the odds of a chemical reaction yielding pure hydrogen and oxygen? Does that make much sense?

I dunno if you could somehow react ethyne and oxygen to produce pure elements, but clearly the most likely reaction, given a highly flammable material and oxygen, is combustion. You could certainly never synthesize sugar out of those starting ingredients; syntheses for sugars are extremely complex and require many, many steps. However, to produce CO2 and H2O, all you need to do is light a match. Just think of the reactants and the possible end products, and decide if what you’re suggesting is sensible or not. H2 and O2 just require a spark to start them reacting to produce H2O, so it’s tough to produce them simultaneously from a chemical reaction.

Generally, chemical reactions proceed from a high energy state to a low energy state, and from an organized state to a disorganized one. Hydrogen and oxygen gases are both higher energy than H2O, which is why you get a big fireball when they react. Sugar is far more organized (less entropic) than C2H2 and O2, since gases are inherently more disorganized than solids. Plus, sugar clearly is in a fairly high energy state, since it can be reacted to produce so much energy. So your sugar synthesis seems highly unlikely; only a huge energy input and very careful control of how the reaction progresses could yield that end product. Plants may do it, but it’s hard to do in a lab.

You need to just look at the reactants and figure out the likely products. Looking at a highly combustible gas and oxygen, the most likely reaction is pretty clear.

5

Doesn’t electrolysis add energy to the system anyway? HS chemistry was a long time ago, and we don’t deal with this in the chemistry I’m in right now.

6

Yes, electrolysis does add energy, and does break bonds, but it is not breaking up a compound in the way combustion with O2 does. There is the classic experiment of placing wires with a DC voltage in a saturated salt solution and creating H2 and O2 out of H2O. You can also create elemental H2 from dissolving zinc in hydrochloric acid (HCl). You can also create elemental carbon from hydrocarbon combustion when the O2 supply is very limited. This is known as soot. However, you can’t create C, H2 and O2 from acetylene combustion because the entropy of diatomic elements is higher than the entropy of oxidized elements. IOW, it would require more energy to create H2 and O2 than H2O, and the resulting products from a chemical reaction will be those with the lowest entropy.

chaoticdonkey, your equation isn’t balanced. You have two carbon atoms on the left side, but four on the right. What’s on one side has to equal what’s on the other side, or you’re in violation of the law of conservation of mass.

Vlad/Igor

7

Remember that if you’re trying to write a spontaneous reaction you will need low energy products. Water is a pretty low energy product. You’ll notice that water seems able to sit around all over our planet while not reacting with much.

Sugars are high energy and would need energy put INTO the system to be created. Plants use light for that energy.

As my organic chem professor said, if water can be evolved from a reaction, it probably will.

8

Vlad/Igor, your equation isn’t balanced. You have 8 oxygen atoms on the left side, but ten on the right. What’s on one side has to equal what’s on the other side, or you’re in violation of the law of conservation of mass.

I had it written down correctly on my piece of paper that I used to balance it.

9

The OP mentioned electrolysis with this reaction. I was assuming that there would be O[sub]2[/sub] present when the electrolysis took place.

To avoid sounding like an asshole and getting pitted, I’ll just state that I’m not quite sure toward whom the end of that was directed, and chalk it up to me not being clear in what I meant.

10

You are correct. It should be:

2 C2H2 + 5 O2 -> 4 CO2 + 2 H20

Where do I pay my fine? :slight_smile:

Vlad/Igor

11

Electrolysis is carried out in an aqueous environment. Dissolving oxygen in water might result in a very low level oxidation of acetylene, if acetylene was also soluble in water. If, however, you were to mix acetylene and oxygen gases, and apply a spark, the results would be more dramatic, but this is not electrolysis. Going back to the OP’s last statement about energy, all non-spontaneous chemical reactions need some amount of energy to get them started. After than, energy released from breaking covalent bonds provides enough energy to break other lower energy bonds, forming products.

Vlad/Igor

12

how do you know what the outcome of a reacton will be? Well up to now, 99% of such knowledge has been rationalisation after the fact, (assuming the possible alternativet reaction products have roughly equal energies). You are right that the outcome will often depend on conditions, catalysts etc. All the respondents that poo-pooed the sugar example forgot that biological organisms do similar unlikely reactions to form sugars all the time (there might even be a bug that digests acetylene).

Only now with powerful computers and software can we even begin to predict in some cases what the products will be from first principles.

In other words, we know that most of the time if you heat a hydrocarbon with oxygen that the product will most likely be mainly carbon dioxide and water, from experience, not any other type of knowledge.

13

Chemical engineers and research chemists do indeed accurately predict products from a given reaction all of the time. I had to do it as well when taking organic chemistry in college back in the 1980s. Products from a given reaction are determined by thermodynamic laws well known in chemistry and physics.

Biological systems do some pretty amazing things with simple carbon compounds, but they do so with enzymes. Simple combustion of acetylene will not produce a sugar, no matter where it takes place. It is possible that somewhere in the world there is an organism that can use acetylene as a carbon source, but it doesn’t reside in the human body.

What other type of knowledge would there be? Should we not rely on observed products to be able to predict products of future reactions?

Vlad/Igor

14

I sorta doubt there is, though, because acetylene isn’t exactly found in quantity. That’s true that what we know about chemistry is based on a posteriori observation (but then, isn’t that what science is?)

At any rate, there is certainly no bug that converts acetylene and oxygen directly into sugar. There’s no essential reason for it to be impossible, but it’s still not something that could be done within a lab, and it’s not something that would involve some simple operation. You could take oxygen and acetylene and react them a hundred trillion times, and you’d never get a single molecule of sugar out of it, because CO2 and H2O are much lower in energy. What happens when you react two chemicals is not random. There is just one possible result of this reaction, and all rationalization aside, the OP ought to understand why his alternatives are impossible if he’s tutoring someone in chemistry.

15

  1. Engineers may accurate predict reactions, but they can only do so when someone has spent a lot time looking at that particular reactions rate, thermodynamics, side products etc. No engineer could ever say what would would happen when say Cu, phenyl bromide and ammonia and formaldehyde would give when mixed together at 400 C as there is nothing remotely around to compare it with.

  2. Enzymes are chemicals too! I am not surewhat the point is?

  3. “Should we not rely on observed products to be able to predict products of future reactions?” that is exactly my point, but (i) it does not help you predict something entirley new (ii) it does not explain why the present reaction gave what it did (iii) and unless you have a vast chemical knowledge would not help you predict whether CH4 + O2 would give mainly CO2 + H2O or say CO + H2O or even CO + H2 (all reactions are possible depending on conditions and catalysts)

My reading of the OP’s question is that without learning thousands of reactions, is there any way of predicting what a particular reaction will give? My point is that at the moment not really, though advances in computers means that soon there will be.

16

  1. Chemistry is at the moment based on a prior observation, but given quantum chemistry and ultrafast computers that will not be so for too long.

  2. Just because CO2 and H2O are lower in energy doesn’t mean that will necessarily be formed, otherwise all life is impossible. Many low temperature reactions do not give the lowest energy products, otherwise your car would have rusted, you would have oxidised, long ago. Many chemical reactions give metastable products, often stable kinetically for thousnads of years.

  3. Actually there are bugs that can convert acetylene to sugars!
    :eek:

e.g from http://etd-submit.etsu.edu/etd/theses/available/etd-1025101-154521/unrestricted/vellorej110801.pdf
“Some strains of rhodococci can grow using gaseous hydrocarbons such as propane, butane, and acetylene.”

life is pretty amazing

17

No, dear, all science is based on observation. Theories and models only exist to fit observations into a larger whole. If computers ever become so sophisticated that they can describe chemical reactions, with certainty, before they occur, it will mean a change in the very conceptions of the science, and likely in every other aspect of human lives. But we’ve a very, very long way from that.

Ok, so there’s bugs that convert hydrocarbons into energy. But care to point out where in the paper it states that they convert them into sugars? Because I only glanced through parts of it, but judicious use of the search tool revealed that the word “sugar” doesn’t appear at all. “Glucose” is only used in reference to its growing medium (which suggests that it’s probably not producing it from environmental chemicals.)

The butane lighter in my pocket can also convert simple hydrocarbons into energy, and as a matter of fact, that’s what I suggested would be the likely result of this reaction.

Again, sugar synthesis is incredibly involved; simply creating glucose from component parts requires an incredible number of steps in the lab, and I’m unaware of a synthesis that starts with acetylene or any simple hydrocarbon - but please enlighten the rest of us if you know of one.

And the fact remains that whatever could, theoretically, be produced from those starting ingredients, there’s only one reaction that will procede easily; it’s so clearly the correct answer to the problem and the most likely product of reaction that any other discussion of it is pretty useless. There’s a lot more to chemistry and to predicting the results of chemical reactions than simply counting the number of atoms and arranging them in various ways to suit you. It doesn’t take much chemistry knowledge to realize that there is no way to simply react acetylene and oxygen to produce a sugar. It’s possible that there is a complex, multi-step synthesis (but you haven’t yet found evidence of it) but if so, the answer would require dozens of steps. There’s such a thing as simply having the sense to see what the likely result of a chemical reaction is. Indeed, combustion wasn’t specified in the homework problem (presumably) but a quick glance at the chemicals in question reveals it to be the only meaningful result. And it won’t produce molecular oxygen and hydrogen.

18

I disagree. Anyone familiar with organic synthetic chemistry would know how each of those compounds behaves in the conditions mentioned, and would be able to predict the products based on that behavior as well as on thermodynamic principles. Many plastics and other materials designed to hold reactive compounds were designed on a drawing board and then synthesized in a lab.

Enzymes are proteins. Enzymes catalyze reactions and make other reactions possible by physically holding compounds together in such a way that they bond, when otherwise such bonding would not occur without the enzyme.

Look up computational organic chemistry, andsome of the people in the field.

That particular reaction and many other simple ones are taught in high school and college first year organic chemistry classes. There are thousands of known reactions that allow you to predict what will happen in a given system, even if the reactants have never been mixed before.

19

Vlad/Igor-Excalibre

  1. I picked the Cu/phenyl bromide example presicely as it is not too far from the chemistry I have been doing (at least part of the time) as part of polymer synthesis for the last eleven years. (you can see my ugly homepage on http://www.ch.cam.ac.uk/CUCL/staff/scm.html ) While in some cases I can hope to predict what reaction products are, in many cases there are twenty or thirty products - most of which I can never work out what they are. Most of the products are not thermodynamic products, but kinetic oddities. Even when I can work out the structure, some times it is “how on earth did that get made?”. Chemistry still throws plenty of surprises.

  2. “Enzymes are proteins. Enzymes catalyze reactions and make other reactions possible by physically holding compounds together in such a way that they bond, when otherwise such bonding would not occur without the enzyme”.

Yes but enzymes are chemicals and do chemical reactions! We can make artificial enzymes from scratch without a protein in sight, There is nothing an enzyme can do that we cant mimic (though often many many times worse). All biology is just chemistry that moves and eats! (don’t tell my biology friends I said that)

  1. “Look up computational organic chemistry, andsome of the people in the field”

I have to tutor computational chemistry and am well aware of its limitations and strengths. As a post-doc I modelled inorganic speciation in high temperature (300 C) water. I also use state of the art quantum modelling programmes such as Jaguar and Gaussian, all the time (admittedly not for following reactions)
Given a very well defined small reaction intermediate and substrate it is often possible to get close to calculating the correct product. However, up to now, we usually do it the other way round. “we got product X, what must the transition state be like?”. This is because the difference in energy between getting product X or Y is small (say several kcal/mol), which is usually the same order of magnitude error in calculating the transition states.

ab initio methods can give very accurate results, but at the moment on small systems (say 3-10 atoms). Larger systems are much much slower to calculate. The other problem with these methods is that you have to decide where you want your molecules to react and assign an approximate reaction path befoe the calculation- in other words you have to know what the reaction is you are studying. If the reaction in reality decides to react back to front from your model then your computor aint going to tell you! (e.g isocyanides can react either on the nitrogen or the carbon atom. If you didn’t know this and only modelled the nitrogen reaction, no computational programme at the moment would ever think “hmmm lets turn the molecule around and try the carbon”)

  1. “That particular reaction and many other simple ones are taught in high school and college first year organic chemistry classes. There are thousands of known reactions that allow you to predict what will happen in a given system, even if the reactants have never been mixed before.”

The combustion of methane with oxygen is a very complicated one, with multiradical pathways, probably 5-10 reactive intermediates. The products are simple, the reaction horribly convoluted. I dont know anyone that could even confidently model that one as most of the rate parameters are not known. It is certainly possible to simplify the model down where one could make reasonable calculations, but change the conditions dramatically and even the most confident modeller would simply say “do the experiment, and let me know the results so I can modify my model”

I feel that you are being fooled by what appear simple reactions giving thermodynamic products. 95% of chemistry is not like that (thank god). Let me put it this way - if could predict what you suggest we could, we could sack 90% of the current chemists, and the Nobel prize commitee would deliver me the chemistry prize every year by post.

  1. “Ok, so there’s bugs that convert hydrocarbons into energy. But care to point out where in the paper it states that they convert them into sugars?”

hmmm if bugs need sugars (for a hundred different uses including recognition and structure) and acetylene is only carbon source they use (e.g. ICI was developing biopol where methane could be used as the only carbon source for their bugs) then acetylene goes to make sugars. I only used that paper as an example of a quick google find. I could could trawl through the scientific literature using our databases to find exactly what you want but I have better things to do

5.“No, dear, all science is based on observation. Theories and models only exist to fit observations into a larger whole.”

My point was that we should in principle be able go to non chemistry-based theories (quantum mechanics) to fully explain chemical reactions. One day (probably fifty years) some one can say what happens if A and B react, and a physicist or mathematician will be able to come up with the answer from scratch, without a chemist saying “in my experience it goes to C”.

“but if so, the answer would require dozens of steps. There’s such a thing as simply having the sense to see what the likely result of a chemical reaction is. Indeed, combustion wasn’t specified in the homework problem (presumably) but a quick glance at the chemicals in question reveals it to be the only meaningful result. And it won’t produce molecular oxygen and hydrogen.”

but combustion takes dozens of steps (CH4 + O2 = CH3 + O2H, CH3 + O2 = CH3O2 etc etc) Change the conditions (e.g oxygen starve) and the main product could be CO, or C etc. You “know” that burning thing gives off CO2, but that is from experience (e.g. prior knowledge). Someone without your sense/experience would not have a clue what it gives. That was the OP point/question. Put it another way, are you sure that CH4 + O2 gives CO2 and H2O at 2000K. Are CO2 and H2O the thermodynamic products at 2000K?